1. Technical Field
The present invention relates to an electrostatic actuator which can be used for an electrostatic driving type ink-jet head and the like, a droplet discharge head, manufacturing methods thereof and a droplet discharge apparatus.
2. Related Art
An electrostatic driving type ink-jet head mounted on a ink-jet recording apparatus can be named as an example of a droplet discharge head that discharges droplets. A typical electrostatic type ink-jet head has an electrostatic actuator part having an individual electrode (a fixed electrode) that is formed on a glass substrate and a silicon vibration plate (a movable electrode) that opposes to the individual electrode with a predetermined gap therebetween. The typical electrostatic type ink-jet head also includes a nozzle substrate in which a plurality of nozzle openings for discharging ink droplets is provided, a discharge chamber that is jointed to the nozzle substrate and communicates with the nozzle opening of the nozzle substrate, and a cavity substrate in which an ink flow passage such as a reservoir is provided. When an electrostatic force is generated in the above-mentioned actuator part, the discharge chamber is pressurized, and ink droplets are discharged from the selected nozzle opening.
In the typical electrostatic actuator, an insulating film is formed on faces that oppose the vibration plate and the individual electrode in order to prevent dielectric breakdown or short-circuit of an insulating film which is formed in the actuator and to secure stability and endurance in the actuator driving. The insulating film is usually made of a thermally-oxidized silicon film. This is because the production of the thermally-oxidized silicon film is relatively easy and it has a fine insulation property. JP-A-2002-19129 is a first example of related art. The first example proposes the electrostatic actuator in which the opposing face of the vibration plate has an insulating film made of a silicon oxide film (hereinafter referred as a “TEOS-SiO2 film”) which is formed by a plasma chemical vapor deposition (CVD) method using tetra-ethoxy-silane (TEOS) as the gaseous basic material. JP-A-8-118626 and JP-A-2003-80708 are a second and a third examples of related art. Where the insulating film is formed only on a one side of the vibration plate, residual electric charges occur in the insulating film of a dielectric body. These residual electric charges deteriorate the stability and the endurance in the actuator driving. To avoid this, the second example proposes the electrostatic actuator in which both faces opposing the vibration plate and the individual electrode respectively have the insulating film. JP-A-2002-46282 is a fourth example of related art. To reduce the residual electric charges, the fourth example proposes the electrostatic actuator in which only the face of the individual electrode side has a double layered electrode protection film consisting of a high volume resistance film and a low volume resistance film. JP-A-2006-271183 is a fifth example of related art. The fifth example proposes the electrostatic actuator in which the insulating film of the actuator is made of a so-called High-k material (a high dielectric constant gate insulating film) whose dielectric constant is higher than that of the silicon oxide thereby the actuator can generate a higher pressure.
Where the thermally oxidized silicon film is used for the insulating film of the electrode in the electrostatic actuator, there is a disadvantage that the application of the thermally oxidized silicon film is limited to a silicon substrate. In the case where the TEOS-SiO2 film is used as the insulating film as described in the first example, the film is contaminated with many carbonaceous impurities because of the nature of the film formation method, CVD. From a result of a driving endurance test, it was found out that there is a problem in the stability of the film such that the TEOS-SiO2 film is abraded away when the vibration plate and the individual electrode repeatedly contact each other.
The second example discloses the electrostatic actuator in which a thermally oxidized film is formed on a face which is situated closer to the vibration plate and a silicon oxide film (hereinafter referred as “a sputter film”) is formed on a face which is situated closer to the individual electrode by sputtering. However the sputter film has a weak dielectric strength so that either the film thickness has to be increased or another better insulation film such as a thermally oxide film has to be further formed in order to prevent the dielectric breakdown of the electrostatic actuator.
According to the third example, both electrodes of the vibration plate and the individual electrode are made from silicon substrates, the insulating film made of a thermally oxidized film is provided not only on the side of the vibration plate but also on the side of the individual electrode, and an insulating film is not formed on a joint face of the silicon substrate. However the silicon substrate is more expensive than the glass substrate, causing a cost problem in the production of the actuator.
The fourth example discloses the electrostatic actuator in which only the face of the individual electrode side has the double layered electrode protection film consisting of a high volume resistance film and a low volume resistance film, and the vibration plate is formed of metal such as molybdenum, tungsten and nickel. However the structure of the electrostatic actuator becomes complicated with such insulating structure and the manufacturing process also becomes complicated. This also causes a cost problem.
The fifth example aims to increase the pressure generated by the actuator by adopting a material whose dielectric constant is higher than that of the silicon oxide for the insulating film of the actuator, which can be explained with reference to the hereunder presented Formula 2. Voltage is needed to be applied between the electrodes in order to drive the actuator. If the dielectric strength of the insulating film provided on the electrode is low, the voltage range applicable to the actuator has to be set lower. Even where the so-called High-k material is used for the insulating film, if the dielectric strength of the High-k material is lower than that of the silicon oxide, it is difficult to increase the pressure which is generated by the actuator (because the applied voltage V has to be set smaller than the value derived by the Formula 2).
Moreover, none of the above-mentioned examples mentions about the combination of the High-k material and the surface protection film concerning the insulating film of the actuator. Particularly, the surface protection film is a member which securely protects the insulating film and the surface protection film is essential for the electrostatic actuator to obtain a long-term driving endurance.
Meanwhile, as for the static driving type ink-jet head having the electrostatic actuators, requests of a higher density and a high speed driving are raised recently for the ink-jet head as a request of higher resolution images is increasing. At the same time, downsizing of the actuator is also requested. To meet such requests, it is important to develop the insulating structure with which the pressure capacity generated by the electrostatic actuator can be increased and the driving stability and the driving endurance can be further improved with a minimum cost.